villi are gathered into bunches, or cotyledons, which in the Sheep (Fig. 45) are convex, and are received into cups of the mucous membrane of the uterus; while in the Cow, on the contrary, they are concave, and fit upon corresponding convexities. of the uterus (Figs. 44 and 46).

Fig. 46. A foetal cotyledon, C2, half separated from the maternal cotyledon, C1, of a Cow. Ch, chorion. U, uterus (after Colin).

No one, probably, would be inclined to object to the association of the orders just mentioned into one great division of the Monodelphia, characterised by its placental structure. But such grouping leaves several important points for discussion. The Elephant, as Professor Owen has shown, has a zonary placenta, and the genus Hyrax has been known since the time of Home to be in like case. Hence, as the elephants are commonly supposed to be closely allied with the Pachydermata, which possess diffuse, non-deciduate placenta, and as Hyrax is now generally, if not universally, admitted into the same order as the Horse, which has a diffuse, non-deciduate placenta, it is argued that placental characters do not indicate natural affinities. A question, indeed, arises, which has not been answered by those who have described the placenta of Elephas and Hyrax. Is the placenta of these animals simply a zone-like arrangement of villi

"Description of the Foetal Membranes and Placenta of the Elephant." Philosophical Transactions, 1857.

or cotyledons, in connection with which no decidua is developed, or is it a true deciduate placenta, resembling that of the Carnivora in the essentials of its internal structure as in its external form? Recent investigation has convinced me, that, in both these animals, the placenta is as truly deciduate as that of a Rodent; so that most unquestionably, if the placental method of classification is to be adopted, both Elephas and Hyrax must go into the same primary division of the Monodelphia as the Rodentia and Carnivora.

But do these facts really present obstacles to the placental system of classification?

So far as the case of the Elephants is concerned, I must confess that I see no difficulty in the way of an arrangement which unites the Proboscidea more closely with the Rodentia than with the Artiodactyla and Perissodactyla, the singular ties which unite the Elephants with the Rodents having been a matter of common remark since the days of Cuvier.

In the absence of any definite knowledge of the placental structure of Rhinoceros and Tapirus, it would, perhaps, be premature to discuss the position of Hyrax, as determined by its placenta ; but if it should eventually appear, as is very probable, that Rhinoceros, like Tapirus and Equus, has a diffuse, non-deciduate placenta, I should have no hesitation in regarding Hyrax as the type of a distinct order of deciduate Monodelphous Mammalia. Hyrax, in fact, hangs by Rhinoceros mainly by the pattern of its molar teeth,--a character which affords anything but a safe guide to affinity in many cases. †

Concerning the placentation of the Sirenia we have no information.

Among the Edentata, the Sloths have presented a cotyledonary placenta, and the Armadillos have been affirmed to possess a discoidal one. I am not aware that the minute structure of the placenta has been examined in either of these groups, but I am

* Home's description of the foetal membranes of the Tapir is very poor, but Bauer's beautiful figures show clearly that the villi are diffuse, as in the Horse. + See, in reference to this point, the late Professor A. Wagner's excellent remarks on Cuvier's exaggeration of the Rhinocerotic affinities of Hyrax, in Schreber's "Säugetiere." Supp. Band, Abth. iv. p. 307.

indebted to Dr. Sharpey for valuable information respecting the placental structure of Manis. The surface of the chorion is covered with fine reticulating ridges, interrupted here and there by round bald spots, giving it an alveolar aspect, something like the inside of the human gall-bladder, but finer. The inner surface of the uterus exhibits fine low ridges or villi, not reticulating quite so much. The chorion presents a band, free from villi, running longitudinally along its concavity, and there is a corresponding bald space on the surface of the uterus. The ridges of the chorion start from the margins of the bald stripe, and run round the ovum. The umbilical vesicle is fusiform. This is clearly a non-deciduate placenta, and the cotyledonary form of that of the Sloth leads me to entertain little doubt that it belongs to the same category.

Admitting all these difficulties and gaps in our information, it still appears to me that the features of the placenta afford by far the best characters which have yet been proposed for classifying the Monodelphous Mammalia, especially if the concomitant modifications of the other foetal appendages, such as the allantois and yelk-sac, be taken into account. And it must be recollected that any difficulties offered by the placental method attach with equal force to the systems of classification based upon cerebral characters which have hitherto been propounded. If any objections, on the ground of general affinities, are offered to the association of Elephas, Hyrax, Felis, and Cercopithecus in the same primary mammalian division of deciduate Monodelphia, they are not removed by constructing that primary division upon other principles, and calling it Gyrencephala.

113

LECTURE VII.

ON THE VERTEBRATE SKULL.

THE STRUCTURE OF THE HUMAN SKULL.

THE human skull is by no means one of the simplest examples of a vertebrate cranium which can be studied, nor is the comprehension of its structure easy; but, as all vertebrate anatomy has started from the investigation of human organization, and the terms osteologists use are derived from those which were originally applied to definite parts of the organism of man, a careful investigation of the fundamental structure of man's skull, becomes an indispensable preliminary to the establishment of anything like a sound comparative nomenclature, or general theory, of the Vertebrate Skull.

Viewed from without (Fig. 47), the human cranium exhibits a multiplicity of bones, united together, partly by sutures, partly by anchylosis, partly by moveable joints, and partly by ligaments; and the study of the boundaries and connections of these bones, apart from any reference to the plan discoverable in the whole construction, is the subject of the topographical anatomist, to whom one constantly observed fact of structure is as valuable as another. The morphologist, on the other hand, without casting the slightest slur upon the valuable labours of the topographer, endeavours to seek out those connections and arrangements of the bony elements of the complex whole which are fundamental, and underlie all the rest; and which are to the craniologist that which physical geography is to the student of geographical science.

I

Perhaps no method of investigating the structure of the skull conduces so much towards the attainment of a clear understanding of this sort of architectural anatomy, as the study of sections, made along planes which have a definite relation to the principal axes of the skull.

Fig. 47.-Diagrammatic side view of a Human Skull.-Fr. Frontal. Pa. Parietal. S.0. Supra-occipital. S.O1. Squama occipitis above the torcular Herophili and lateral sinuses. As. Alisphenoid. Sq. Portio squamosa of the temporal bone. M. Mastoid process and pars mastoidea. Ty. Tympanic. St. Styloid process. Na. Nasal. L. Lachrymal. Ju. Jugal, or Malar. Pmx. Premaxilla. Mr. Maxilla. Mn. Mandible. Hy. Hyoid. m. Malleus. i. incus. [These letters will bear the same signification throughout the series of figures of crania.]